1. Field of the Invention
The present invention relates to apparatus for and methods of drawing patterns using a charged particle beam. More specifically, the present invention relates to charged particle beam lithography apparatus and methods which make dose corrections in drawing patterns by the use of a charged particle beam.
2. Description of the Related Art
With conventional electron beam lithography apparatus employing a variable shaped electron beam, dose is controlled to make corrections to pattern proximity effects. With the conventional apparatus, in order to reduce the time required to transfer pattern drawing data from an external storage unit to a drawing control circuit and to save the resource of an internal storage unit, pattern drawing data is compressed by a method using such a data structure as described below.
Consider a case where group patterns 60, each composed of two elemental FIGS. 61 and 62, are arrayed in four rows and four columns as shown in FIG. 1. Drawing data is represented, as shown in FIG. 2, by a placement data group, a figure data group, and a dose control data group.
The figure data group contains a group of group patterns each of which is a collection of elemental figures each constructing a certain pattern. The figure data is a set of data describing the code which means a kind of figure, the irradiation position (X.sub.m, Y.sub.m), and the size (H.sub.m, W.sub.m) of each of elemental figures constructing a desired pattern.
The placement data group is placement information of the group patterns. The placement data for a group pattern i comprises a set of data on the positions (X.sub.xi, Y.sub.yi) of a reference point of placement, the repetition counts N.sub.xi, N.sub.yi (in the example of FIG. 1, N.sub.xi =N.sub.yi =4) of the group pattern, the repetition pitches P.sub.xi, P.sub.yi of the group pattern, the number Ni of elemental figures in the group pattern, the starting address PFi of the storage area storing the figure data, and the starting address PDi of the storage area storing dose control data for the figure data.
The dose control data group is a set of data describing the dose t.sub.m of each unit figure.
In the case where one group pattern is arranged repeatedly, the definition of figure data representing the group pattern, group pattern placement data, and dose control data for the figure data will be adequate. Thus, the use of the above method will permit compression of pattern drawing data.
With the above method, if the size of an elemental figure is larger than the broadening of the backscattering of an irradiation electron beam, the original elemental figure must be subdivided into elemental figures each of which is much smaller than the broadening of the backscattering, and dose control data must be described for each of the subdivided elemental figures. This will be described with reference to FIGS. 3A through 3C.
When, as shown in FIG. 3a, an electron beam falls on a point of irradiation 81, i.e., a point O, the range 81 of the distribution of backscattered electron will be within a circle with a radius of .sigma.(=3.sigma..sub.b) and with a center at the point O, where .sigma..sub.b represents the broadening of the backscattering. If, as shown in FIG. 3B, an another desired elemental FIG. 83 were drawn, with its dose controlled, within the circle 82, the total dose would differ from a predetermined amount. In order to perform drawing with the total dose controlled, it is necessary that, as shown in FIG. 3C, a unit FIG. 84 is much smaller than the circle, and the dose for the unit FIG. 84 is controlled taking into account the dose by backscattering caused in drawing other elemental figures within the circle 82.
As described above, when the size of an elemental figure constructing a pattern is large, the compression of drawing data, which is possible with the conventional method, is impossible and moreover an increase in the quantity of drawing data results. That is, a problem with the conventional method is that the compression of drawing data is insufficient in making dose corrections. In addition, insufficient data compression by the conventional method will make data transfer time long, decrease the operating ratio of an electron beam lithography apparatus and in its turn decrease its drawing throughput.
As described above, the conventional method of compression of drawing data cannot attain sufficient data compression in making corrections to proximity effect. Thus, the drawing data transfer time inevitably becomes long, which reduces the operating ratio of the charged particle beam drawing apparatus.
Techniques related to the present invention include T. Abe et al., "Representative Figure Method for Proximity Effect Correction," Japanese Journal of Applied Physics, Vo. 30, No. 3B, March, 1991, pp. 258-351, M. Parikh, "Correction to Proximity Effects in Electron Beam Lithography (I. Theory; pp. 4371, II. Implementation; pp. 4378 and III. Experiments; pp. 4383)," J. Appl. Phys. 50(6). June 1979, Y. Machida et al., Japanese Examined Publication, Feb. 23, 1988, T. Abe et al., "Proximity effect correction for high-voltage electron beam lithography," J. Appl. Phys. 65(11), 1 June 1989, and J. M. Pavkovich, "Proximity effect correction calculation by the integral equation approximate solution method," J. Vac. Technol. B4(1), January/February 1986.